1. Field of the Invention
The present invention relates to an electrode and an electrolyte for use in the preparation of a nitrogen trifluoride gas, and a preparation method of the nitrogen trifluoride gas by the use of the electrode and the electrolyte.
More specifically, it relates to an electrode and an electrolyte for use in the preparation of a nitrogen trifluoride gas by the electrolysis of an ammonium fluoride (NH4F)-hydrogen fluoride (HF)-containing molten salt, and a preparation method of the nitrogen trifluoride gas by the use of the above electrode and electrolyte.
2. Description of the Related Art
With the drastic advancement of electronic industries in recent years, the density and the performance of semiconductor elements have been heightened, and the production of very large-scale integrated circuits has been increased. In consequence, a high-purity nitrogen trifluoride gas has been required as a gas for dry etching for use in a preparation process of integrated circuits and as a gas for a cleaner of a CVD apparatus.
The preparation methods of the nitrogen trifluoride (hereinafter abbreviated to xe2x80x9cNF3xe2x80x9d) gas can be roughly classified into a chemical method and an electrolysis method. The chemical method comprises a first step in which a fluorine (hereinafter abbreviated to xe2x80x9cF2xe2x80x9d) gas is produced, and a second step in which the thus obtained F2 gas is reacted with a raw material containing nitrogen to thereby prepare the NF3 gas. On the other hand, the electrolysis method comprises preparing a non-aqueous molten salt containing nitrogen component and fluorine component as an electrolyte, and then electrolyzing the electrolyte to thereby prepare the NF3 gas.
As compared with the chemical method, the electrolysis method has an advantage that the NF3 gas can be prepared in a high yield in one step.
The chemical method uses an F2 raw material containing a large amount of carbon tetrafluoride (hereinafter abbreviated to xe2x80x9cCF4 xe2x80x9d), and hence the NF3 gas is inevitably contaminated with the large amount of CF4. However, this CF4 is extremely similar to NF3 in physical properties, and in order to obtain the high-purity NF3 gas, it is unavoidable to apply an advanced purification technique, which is industrially costly. On the contrary, in the electrolysis method, CF4 is scarcely produced or entrained in a synthetic process, and hence, it has a merit that the high-pure NF3 gas can be easily obtained.
The outline of an industrial synthesis of the NF3 gas by the electrolysis method is as follows. As an electrolyte, there is used an NH4F-HF molten salt comprising ammonia, acidified ammonium fluoride (NH4HF2) and anhydrous hydrogen fluoride (HF). Using an anode made of a metallic material electrolytes the above molten salt. The NF3 gas is generated on the anode, thereby obtaining the NF3 gas containing impurities. After a purifying operation, the purity of the NF3 gas is in excess of 99.99 vol %.
The metallic material, which is most suitable for the anode, is nickel. When another metal is used, passivation occurs owing to the formation of the oxide film on the anode surface, so that current does not flow, or it is vigorously dissolved into the electrolyte. Even nickel is slightly dissolved, and hence the electrode is consumed. In consequence, in an industrial production, it is required to often replace the electrode, and it is also unavoidable to exchange the electrolyte contaminated with nickel salts produced by the dissolution.
The electrolysis method is an excellent technique for easily obtaining the high-pure nitrogen trifluoride gas, but it has been an industrially important theme to inhibit the dissolution of the anode.
For this theme, various electrode materials and electrolytes for inhibiting the dissolution of the electrode have been investigated.
The present inventors have intensively investigated the differences of dissolution behavior between nickel and other metals in order to achieve the inhibition of the dissolution. As a result, it has been found that the surface of nickel in a highly oxidative state is covered by a stable conductive oxyfluoride at the time of electrolysis in the aforementioned molten salt, and the exchange of electrons is carried out via the resultant film between the electrode and an electrolyte, so that nickel is less dissolved than the other metals, and a passivation does not occur and therefore electrolysis can be performed. It has been suggested that, for the purpose of positively promoting the production of the oxyfluoride on the surface of the electrode, an oxide of nickel is mixed with a nickel dispersed plating or a nickel powder, followed by sintering, to reduce the amount of dissolved nickel (Japanese Patent Application Laid-open No. 225976/1996). However, further intensive investigation has been conducted to seek for an easier technique, and as a result, it has been found that the amount of dissolved nickel can be reduced by controlling an Si content present in the electrode to 0.07 wt % or less, introducing a transition metal into the nickel electrode, and allowing a certain amount or more of the transition metal to exist in the electrolyte, and in consequence, the present invention has been completed.
That is to say, the present invention is directed to an electrode for electrolyzing an electrolyte comprising an ammonium fluoride (NH4F)-hydrogen fluoride (HF)-containing molten salt, a composition ratio (HF/NH4F) being in a range of 1 to 3, said electrode comprising nickel in which an Si content is 0.07 wt % or less, a transition metal other than nickel being added to the nickel electrode. Furthermore, it is directed to a preparation method of a nitrogen trifluoride gas by the use of the above electrode and/or the electrolyte containing a transition metal.
The method of the present invention is an epoch-making invention in which the amount of dissolved nickel can be remarkably reduced without changing a conventional electrolysis process. In consequence, the frequency of replacing the electrode or the electrolyte can be decreased to half or less of a conventional case, and cost can also be reduced. The effects of the present invention are extremely large in industrial production.